Safety cooling installation for a water nuclear reactor

ABSTRACT

The invention concerns a safety cooling installation for the water reactor of a nuclear power station, the installation notably incorporating a reservoir for storing a cooling liquid, the said reservoir being located outside the containment which encloses the reactor circuit, the installation being provided with aspiration and discharging means arranged to aspirate the liquid from the reservoir and to discharge it into the circuit, there being provided a passive, maintenance free means to recover and conduct the liquid and containment water, the said means being arranged to recover the liquid and the water from the reactor in the lower part of the containment by non gravitational flow in the event of a break in the reactor&#39;s circuit.

BACKGROUND OF THE INVENTION

The present invention concerns a safety cooling installation for thewater reactor of nuclear power stations.

It is known that a nuclear power station should be equipped with safetyor safeguarding circuits whose function is to maintain, with a highlevel of reliability, the power station--and, in particular, thereactor--in a safe and stable state in the event of an accident. Inparticular, power stations which employ water as the functional coolingfluid are equipped with circuits which make it possible, in the eventthe primary circuit is depressurized, to inject water under pressure inorder to cool the fuel and prevent it from melting and, in the majorityof cases, the stations are provided with circuits which make it possibleto inject a fine spray of water into the object of this spraying beingto reduce the pressure in the containment when the latter is subjectedto a sudden increase in pressure subsequent to a rupture in thereactor's circuit where extremely high pressures exist, namely pressuresof the order of 150 bars.

The water injected by these safeguarding circuits is furnished by one ormore reservoirs generally located outside the containment enclosing thereactor circuit. In the case of a rupture of the pressure barrier of thereactor circuit, the said safety or safeguarding circuits enter intoservice automatically. The water injected by the safety circuits mixedwith the reactor fluid flows to the lower part of the containment,called the sump. Because of the enormously high rates at which it isrequired to inject the water, the external reservoirs are rapidlyemptied, and, because it is unacceptable to operate with an open circuitdue to the large amount of radioactive contaminants in the waterrecovered in the sump, it is necessary to recycle this water in such away as to keep the reactor fuel completely immersed and thus prevent itfrom melting.

Passage from the "injection" phase to the "recirculation" phase isgenerally effected automatically after a period which can vary from 20minutes to several hours, depending on the size of the break. Safetyregulations make it necessary to assume that the water located in thecontainment's sump is at saturation, that is, under conditions of bulkboiling at 120°-130° C. Such conditions impose very severe constraintson the selection and disposition of the pumps serving to extract thewater from the containment, and also on the arrangement of the pipesconnecting the sump to the said aspirating pumps. In effect, the watershould not attain this state of saturation at any point in the saidpipes, the consequence of such a saturation being the formation of vaporlocks which would block the recirculation and hence produce cavitationand rapid destruction of the pumps.

According to present-day design, the pumps are installed in a very deepauxiliary building which is, for example, located at depths of 5 to 10meters below the sump and in such a way as to obtain the necessarypressure for the aspiration, taking into account the load losses in thepipeline connecting the sump to the pumps. Such a design may posedifficult civil engineering problems, depending on the nature of theterrain and the seismic conditions of the site.

Another solution consists in the use of vertical pumps mounted in acylindrical housing let directly into the structure of the auxiliarybuilding, the length of this housing making it possible to obtain thepressure necessary for aspiration without the need for a similarly deepbuilding.

It is evident that these designs, while being technically acceptable,are extremely costly and pose very large installation problems.

Use has also been made of special pumps which are mounted directlyinside the containment's sump. These extraction pumps make it possibleto provide the necessary aspiration pressure to the safeguarding pumps.These pumps, which are installed at the bottom of the sump, are actuatedeither by an immersed motor or by a "dry" motor located at a heightsufficient for it not to be immersed. Such solutions not only requirespecially designed and qualified active components, that is, componentsrequiring electrical energy in order to function and also requiringregular maintenance, but are, moreover, in contradiction with thepresent safety regulations which require a design which makes possiblethe maintenance and repair of the active equipment during the periodfollowing the accident. Such maintenance is impossible with the abovearrangements, as the containment is absolutely inaccessible for a periodwhich could be many years on account of the very high radioactivitylevel reigning in it.

Modifying power stations equipped with containment recirculation pump insuch a way as to bring them to a state of reliability conforming topresent-day regulations poses extremely complex problems andnon-negligible risks, resulting in the need for digging an excavationwhich is large by comparison with the containment. Such a modificationwould also shut down production for several months, entailing, inaddition, enormous financial losses.

SUMMARY OF THE INVENTION

The objects of the invention is to provide a safety installation thatmeets present day safety regulations by the use of means to recover andconduct water from the containment sump which, without using gravity,are totally passive and do not require maintenance nor do they requirethe construction of a deep auxiliary building, thus eliminating allrisks of endangering the stability of existing structures, in the caseof an old power station, and greatly simplifying the civil engineering,in the case of a new power station and also eliminating the possibilityof a non controlled contamination of ground water by having nocontainment penetration below maximum spillage water level.

To this end, in accordance with the invention, the installationincorporates a reservoir for storing a cooling liquid such as water andlocated outside the containment which encloses the circuit of thereactor, the installation also incorporates means for aspirating anddischarging, arranged at least to aspirate the liquid from the reservoirand to discharge the said liquid into the said circuit, a first liquidconduit which connects the reservoir to the said means, a second liquidconduit leaving the same means and terminating in the circuit of thereactor, there being provided means to recover and to convey the saidliquid and the containment water and arranged to recover the said liquidand the water from the reactor, in the event of a break in the circuitof the latter, in the lower part of the containment after the saidliquid has at least passed into the reactor's circuit, and conduct thesaid liquid and water by non gravitational flow to the said means foraspirating and discharging the cooling liquid, there being provided afirst pipe-line for transferring liquid and containment water, the saidpipe-line connecting the said means for recovering and conducting theliquid and containment water to the above-mentioned first conduit andbeing located upstream of the means of aspiration and discharging, theinstallation also incorporating a second pipe-line for transferring theliquid and containment water, the second pipe-line connecting theabove-mentioned second conduit downstream of the aspiration anddischarge means to the said means for recovering and conducting theliquid and the containment water, there also being incorporated valvesarranged for permitting or stopping the flow of the liquid and the waterwhich pass through the said conduits and the said pipe-lines.

According to one embodiment of the invention, the above-mentionedaspiration and discharging means consists of a pump, and theabove-mentioned means for recovering and conducting the liquid and thecontainment water consists of an ejector.

According to an advantageous embodiment of the invention, theinstallation incorporates, in the upper part of the containment, atleast one spray manifold arranged to project cooling liquid to theinterior of the said containment and over at least a large portion ofthe volume it occupies, the said manifold being supplied with coolingliquid by the above-mentioned pump, the installation also being providedwith a pipe-line which connects the second conduit, downstream of thepump, to the spray manifold.

According to an especially advantageous embodiment of the invention, theinstallation incorporates a hermetically sealed tank which is insertedin the first pipe-line connecting the ejector to the first conduit andwhose axis is located in an essentially vertical plane, the said tankbeing provided with an opening let in at the top to which is connectedthe pipe-line originating from the ejector, and provided at the bottomwith an opening to which is connected the pipe-line connecting the firstconduit, the said tank additionally incorporating a vent-pipe leading tothe containment and arranged to allow contaminated gases or vapors,which accumulate in the top part of the tank when the above-mentionedpipe-lines are filled with cooling liquid and water, to escape to theinterior of the said containment.

According to a particularly advantageous embodiment of the invention,the installation incorporates a third conduit which, downstream of thepump, connects the second conduit to the reservoir, the installationalso incorporating a valve which is normally closed when theinstallation is in use, the valve being associated with the said thirdconduit, and being arranged, in particular, to enable the functioning ofthe pump to be verified or to fill the reservoir when the latter isempty.

BRIEF DESCRIPTION OF THE DRAWINGS

Other details and special features of the invention will become evidentfrom the description of the appended schematic drawing which represents,by way of non-limiting example, a particular embodiment of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

The safety installation constructed in accordance with the presentinvention and illustrated by the single appended figure incorporates areservoir 1 for storing a cooling liquid 2 such as water, a pump 3, aconduit 4 which connects the reservoir 1 to the pump 3 and a conduit 5which leaves the pump 3 and terminates in the water reactor circuit (notshown) of a power station. The pump 3 makes it possible, in particular,to aspirate the cooling liquid 2 from the reservoir 1 and to dischargethe said liquid, or part of it, via the conduits 4 and 5, into thecircuit of the reactor. As is the case with the pump 3, the reservoir 1is located outside the sealed containment 6 which encloses the reactorcircuit. A spraying manifold 7 is located in the upper part of thecontainment 6. It consists of one or more distributing tubes 16 equippedwith regularly spaced spray nozzles 17. The said manifold is designed toproject part of the cooling liquid 2 of reservoir 1 into the interior ofthe containment over at least a large part of the volume occupied by thelatter, the said manifold being supplied with cooling liquid by the pump3, a pipe-line 8 connecting conduit 5 to the manifold, downstream of thesaid pump 3.

Not only does this double injection of water make it possibleefficiently to cool the circuit of the reactor and thus prevent thelatter from melting, but it also serves to reduce the pressure in thecontainment by reducing the temperature in the latter when a breakoccurs in the reactor's circuit.

In order to pass from the injection phase to the recirculation phase ofthe cooling liquid, the installation is provided with an ejector 9, thesaid ejector making it possible to recover the cooling liquid fallinginto the sump 10 of the containment 6, the cooling liquid being mixedwith water from the reactor in the event of a break or rupture in thecircuit of the latter, and conduct this liquid mixture by nongravitational flow towards the pump 3 after the liquid 2 has passed intothe circuit of the reactor. To this end, a pipe-line 11 connects theejector 9 to the conduit 4 upstream of pump 3, and a pipe-line 12connects the conduit 5, downstream of pump 3, to the ejector 9. A valve13, which is incorporated in the pipe-line 12, and two valves 14 and 15which are incorporated in the pipe-line 11, are arranged to permit orblock the flow of cooling liquid and of the water coming from thecircuit of the reactor.

In order to reactivate the ejector 9 and/or in order to raise thepressure of the liquid circulating in the pipe-lines 11, 12 associatedwith the ejector, to separate gases from the liquid the installation isprovided with a hermetically sealed tank 18 whose axis is vertical. Thistank 18, which is inserted in the pipe-line 11 between the valves 14 and15, has, let into its upper part, an opening 19 to which is connectedthe part of the pipe-line 11 coming from the ejector 9, the lower partof the tank also being provided with an opening 20 to which is connectedthe part of the pipe-line 11 connected to the conduit 4. A vent pipe 21leading to the containment 6 is set up to allow the gas or contaminatedvapors, which accumulate in the top part of the tank when the ejector'spipe-lines are filled with cooling liquid and water, to escape to theinterior of the containment. This tank 18 could, of course, be replacedby a normal pipe-line, that is, a pipe-line 11 could be provided withoutthe tank 18. In this case, restarting of the ejector is effected via theprincipal reservoir 1.

Means 22, inserted in the conduit 5 downstream of the pump 3, areprovided to contain a cooling fluid and to permit an exchange of heatbetween this cooling fluid and the cooling liquid and reactor water tobe cooled, after the liquid, and, if such be the case, the reactorwater, have passed through the reactor circuit. The installation isprovided with a non-return valve 23 incorporated in the conduit 4, thevalve allowing cooling liquid and water to pass towards the pump 3 andprevents them from passing in the reverse direction, that is, itprevents them from discharging into the reservoir 1.

In order, in particular, to verify that the pump 3 is functioning or torefill the reservoir 1 when the latter is empty, there is provided aconduit 24 which connects the conduit 5, downstream of the pump 2, tothe reservoir 1, and also a valve 25 which is inserted in the saidconduit 24 and which is normally closed when the installation isfunctioning.

In addition to the above-mentioned advantages, the safety coolinginstallation of the invention has the following advantages when comparedwith conventional installations:

(1) The ejector has no problem in pumping water loaded with debris suchas pieces of concrete, heat-insulating rock wool, etc., with no risk ofdestruction.

(2) The ejector can function at a very low inlet pressure and withstandscavitation which could either result in a partial blockage of the inletor in too low a water level in the sump.

(3) The ejector can easily be designed to withstand an earthquake.

(4) The ejector may be installed at the location most appropriate as faras operation and protection against possible missiles are concerned.

(5) In the event of the aspiration being blocked, unblocking may beeffected by water pressure, by reversing the direction of flow.

(6) Since the size of the safeguarding pumps are conditioed by theinjection phase, the said pumps having a large margin during therecirculation phase, and a fraction of their flow can be used fordriving the injectors.

Some installations are not equipped with a spray circuit within thecontainment and, on the other hand, where such a circuit exists, it ispreferable to stop the spraying as soon as possible by closing the valve26 in such a way as to limit the dispersion of contaminated water overthe equipment and the structures.

Consequently, in the most probable case of a small rupture in theprimary circuit, the recirculation flow-rate is very small when thespraying function is absent, which greatly reduces the rate at which theheat stored in the sump water is removed.

In addition, if so desired, the installation of the invention makes itpossible to maintain a high rate of recirculation, thus ensuring rapidcooling of the sump water. This has a direct effect on the temperatureof the containment and, consequently, on the pressure reigning withinit.

The way in which the safety cooling installation of the inventionfunctions may be easily understood by referring to the appended figure.

Subsequent to a break which leads to a depressurization of the reactorcircuit and, as a result, to an increase of pressure in the containment6, the one or more pumps 3 are put into service automatically and injectwater under adequate pressure into the circuit of the reactor and, ifsuch be the case, into the interior of the containment via the conduits4, 5 and 8, the water being drawn from the storage reservoir 1. When thewater level in the reservoir 1 reaches a predetermined lower limit, theejector circuit is put manually or automatically into operation byprogressively opening the valves 13 and 14. The ejector 9 takes back thesump water, that is, the injection water coming from the storagereservoir 1 and the functional water coming from the so-called reactorcircuit. After a certain time, the ejector circuit is completely filledwith water, the air containing the contaminating radioactive productshaving been expelled to the interior of the containment via theevacuation vent-pipe 21 belonging to the tank 18. The ejector 9 also hasthe effect of raising the pressure of the water circulating in theconduits, the pressure of the water leaving the tank 18 via the sectionof tube 11 being appreciably higher than that reigning in the conduit 4on the reservoir side. The valve 15 is then opened, the pump 3 thenbeing supplied with water coming from the sump 10 and the ejector 9.This water, which is radioactive, is not discharged into the reservoir 1because the non-return valve 23 blocks the flow of water in thisdirection. The heat exchanger 22, which is cooled by a different watercircuit, makes it possible rapidly to reduce the temperature of thewater put back into circulation. The water is next re-injected into thereactor circuit via the conduit 5 and eventually to the interior of thecontainment via the conduit 8. Part of the sump water likewise passesthrough the pipe-line 12 and is conveyed towards the ejector 9. Forbetter understanding the circuit associated with the ejector 9 is shownby the slightly thicker lines.

The return conduit 24 to the reservoir 1 is normally closed by the valve25 and is only used to verify that the one or more pumps 3 arefunctioning, for filling the reservoir 1 or, if need be, for otherpurposes. In addition, in the event of a momentary stoppage of thecircuit as, for example, during an interruption of the electric powersupply, the ejector may be re-energized either by closing the valves 13,14 and 15 and causing the injection to restart from the reservoir 1 andrecommencing the above-defined sequence, or by using the reserve ofwater contained in the priming tank 18 insofar as the latter is filled.

It will be noted that the water coming from the so-called reactorcircuit, which generally contains a certain amount of boric acid, andthe cooling liquid coming from the storage reservoir, which likewisenormally consists of water, and more particularly borated water, flow inthe installation in a closed circuit. This prevents any possiblecontamination from occurring outside the containment and theabove-mentioned installation.

It should be understood that the present invention is in no way limitedto the embodiments described above, and modifications may be madewithout departing from the scope of the present patent.

We claim:
 1. In a pressurized water nuclear power reactor, a safetycooling system comprising:a reservoir for storing a cooling liquid,located outside the containment which encloses the circuit of thereactor; a pump, arranged at least to aspirate the liquid from thereservoir and to discharge said liquid into said circuit; a first liquidconduit which connects the reservoir to said pump; a second liquidconduit leaving said pump and terminating in the circuit of the reactor,an ejector to recover and to convey said liquid and the containmentwater, arranged to receover said liquid and the water from the reactorin the event of a break in the circuit of the reactor, in the lower partof the containment after said liquid has at least passed into thereactor's circuit, and conduct said liquid and water by nongravitational flow to said pump; a first pipe-line for transferringliquid and containment water, connecting said ejector to said firstconduit and being located upstream of the pump; valve means in saidfirst pipe-line for permitting or stopping flow therethrough; a secondpipe-line for transferring the liquid and containment water, connectingsaid second conduit downstream of the said pump to said ejector, andvalve means in said second pipe-line for permitting or stopping flowtherethrough.
 2. In the pressurized water nuclear power reactor asdefined in claim 1, the improvement wherein in the upper part of thecontainment, there is provided at least one spray manifold arranged toproject cooling liquid into the interior of the said containment andover at least a large part of the volume occupied by said containment,said manifold being supplied with cooling liquid by said pump, therealso being provided a pipe-line which connects the second conduit,downstream of the pump, to the spray manifold.
 3. In the pressurizedwater nuclear power reactor as defined in claim 1, the improvementwherein a hermetically sealed tank is incorporated in the firstpipe-line connecting the pump to the first conduit, the axis of saidtank being located in an essentially vertical plane the upper part ofsaid tank being provided with an opening to which is connected thepipe-line coming from the ejector, and the lower part of the tank isprovided with an opening to which is connected the pipe-line connectedto the first conduit.
 4. In the pressurized water nuclear power reactoras defined in claim 3, the improvement wherein said tank is providedwith a vent pipe which leads into the containment and which is arrangedto allow gas or contaminated vapors, which accummulate in the top partof the tank, when the said pipe-lines are filled with cooling liquid andwater, to escape into the interior of said containment.
 5. In thepressurized water nuclear power reactor as defined in claim 1, theimprovement comprising means to contain a cooling fluid and to permit anexchange of heat between, on the one hand, the cooling liquid and thereactor water to be cooled after said liquid has at least passed intothe reactor circuit, and, on the other hand, the cooling fluid.
 6. Inthe pressurized water nuclear power reactor as defined in claim 1, theimprovement comprising a non-return valve associated with the firstconduit connecting the reservoir to the pump and arranged to allow thecooling liquid and water to pass to the pump and to prevent passagethereof in the opposite direction.
 7. In the pressurized water nuclearpower reactor as defined in claim 1, the improvement wherein a thirdconduit is provided which connects the second conduit, downstream of thepump, to the reservoir, and a valve is provided in said third conduit,said valve being normally closed while the safety cooling system is inuse, said valve being arranged to enable the functioning of the pump tobe checked or to fill the reservoir when the latter is empty.
 8. In thepressurized water nuclear power reactor as defined in claim 1, theimprovement wherein the cooling liquid and water coming from the reactorcirculate in the safety cooling system in a closed circuit.
 9. In thepressurized water nuclear power reactor as defined in claim 1, theimprovement wherein said cooling liquid is water.